In most hydraulic tensioners currently in use, a ferrous metal plunger is arranged to protrude slidably from a tensioner body composed of a metal such as cast iron, aluminum alloy or the like. The metal tensioner body has a large mass. Reduction of the overall weight of the tensioner can be achieved by utilizing a tensioner body composed of a synthetic resin. The tensioner in which the tensioner body is composed of a synthetic resin is generally known as a “plastic tensioner.” An example of a plastic tensioner is described in U.S. Pat. No. 5,967,921, granted Oct. 19, 1999. A typical plastic tensioner is structurally complicated, and is difficult to manufacture.
To simplify the plastic tensioner and facilitate its manufacture, certain measures have been taken in conventional plastic tensioners. In the conventional plastic tensioner, a cylindrical metal tube is insert-molded in a hole in the synthetic resin tensioner body, forming a liner in which the plunger of the tensioner is slidable.
As in the case of a hydraulic tensioner having a metal body, a coil spring, in compression between a closed end of the plunger and the bottom of the hole in the tensioner body, biases the plunger in the protruding direction. The plunger, and the lined hole in the tensioner body cooperatively form a high-pressure oil chamber which receives oil under pressure, through a one-way check valve, from an oil reservoir formed in the tensioner body and communicating with a oil port in the engine block on which the tensioner is mounted.
In the plastic tensioner, bolts, for mounting the tensioner body on the engine block, extend through metal bushings which are insert-molded into the tensioner.
In the operation of the tensioner, the high pressure oil chamber is always filled with oil supplied through the oil reservoir. When the timing chain loosens, the biasing force exerted by the compression spring moves the plunger in the protruding direction. At the same time, the check valve opens, allowing oil to flows into the high pressure oil chamber from the oil reservoir. When tension is reestablished in the timing chain, and also when additional tension is imparted to the chain, for example by an impact force, the plunger is pushed into the tensioner body, but the check valve closes, limiting retracting movement of the plunger.
As shown in FIG. 13, in the conventional plastic hydraulic tensioner, a bolt 22 extends, through a metal bushing 26 insert-molded into the tensioner body 2, and is threaded into an engine block 21. The bushing should have an outer diameter at least as large as the diameter of the head of the bolt in order to avoid buckling of the bushing and to maintain the tensioner body stably fastened to the engine block. Two such insert-molded bushings and mounting bolts are provided.
The bushing 26 has a smooth outer circumferential surface. Vibrations of the engine can result in rotation of the bushing, which can, in turn, cause rotation of the bolt 22, causing the bolt to loosen. Even very feeble vibrations can result in loosening of the mounting bolts in this manner. Furthermore, insert-molding does not necessarily fix the tensioner body 2 firmly to the bushings. If the fit of tensioner body to the bushings becomes loose, the tensioner can separate from the engine block 21, as shown in FIG. 14, even while the bushings 26 and the bolts 22 are fixed to the engine block 21. The pressure of oil in the oil reservoir also exerts a force on the tensioner body (indicated by arrows in FIG. 13), urging the tensioner body away from the engine block as shown in FIG. 14. When the tensioner floats away from the engine block, a clearance d is created, through which oil can leak from the oil reservoir 16. When such leakage occurs, an adequate amount of oil may not be supplied to the high pressure oil chamber of the tensioner, and backlash in the chain drive can occur, resulting in the generation of backlash noise.
The invention addresses the above-mentioned problems by preventing an insert-molded metal bushing from rotating or becoming disconnected from the tensioner housing as a result of engine vibration. It ensures that the tensioner body remains firmly engaged with the tensioner mounting surface on the engine block on which the tensioner is mounted, and prevents leakage of oil that can result from separation of the tensioner body from the engine block.